The invention relates generally to the field of electromyography and is more particularly directed to means for the clinical detection and indication of abnormal electromyographic response of the masticatory muscles.
Electromyography is concerned with the measurement of electrical signals which accompany skeletal muscle activity. Laboratory apparatus for detecting and recording these signals has been available to medical researchers for some time and has enabled them to gain insight into the etiology of various neuromuscular disorders. Electromyography also shows promise as a useful clinical tool in the diagnosis and treatment of certain neuromuscular disorders.
In particular, certain jaw disorders are known to affect the electrical activity of the masticatory muscles. In the muscles spanning the temple area and the upper reaches of the jaw--the temporalis and masseter--an electrical signal is generated when a person clenches the teeth. It is a well documented phenomenon that when the masticatory muscles are contracted in this way and the chin is tapped lightly, there will be a momentary cessation of electrical activity. This momentary cessation is known as the Silent Period.
The studies of Bessette and Shatkin, Journal of Plastic Reconstructive Surgery, Vol. 64, pp. 232-238, and of Skiba and Laskin, Journal of Dental Research, Vol. 60, pp. 699-706, among others, have shown that persons with temporomandibular joint dysfunction or myofascial pain dysfunction exhibit prolonged Silent Periods. Moreover, the degree of prolongation appears to be directly related to the severity of the symptoms.
These studies suggest that measurement of the Silent Period could be a useful tool for the clinician in the diagnosis and treatment of the above-mentioned disorders, to be used along with such other established modalities as muscle palpation, maximum jaw opening, and the patient's own subjective symptoms.
Although the Silent Period has been referred to as though a well defined quantity, this represents an idealization. In fact a typical electromyographic waveform displays erratic spikes and fluctuations. Short periods of reduced electrical signal interspersed between the spikes can be confused with true Silent Periods associated with reflexive muscle response. Moreover, even true Silent Periods do not always have a clearly delineated beginning, but rather sometimes follow upon the tail of a gradually decaying waveform from a peak of electrical activity. And as with any other weak electrical signal, noise interference will sometimes obscure important features. Experienced researchers will be able to recognize features of significance in a complex electromyographic waveform, but the typical clinician will not have the requisite interpretive training to apply electromyographic analysis as a reliable diagnostic tool.
Moreover, known electromyographic apparatus used in medical research is generally too expensive and too impractical to find widespread use in clinical applications. An attempt to develop an instrument more suitable for clinical use has been made by Yamada et al., IEEE Transactions on Biomedical Engineering, Vol. BME-27, No. 7 (July 1980). Although their instrument shows a great simplification over laboratory electromyographic apparatus, it is still more complicated than need be. The Yamada instrument measures both the duration and latency of the Silent Period. "Latency" is understood to mean that period of time between application of the inducing stimulus and the onset of the Silent Period. To measure latency, the Yamada instrument includes a memory module for "memorizing" a digital data stream representative of electrical activity. The data may then be retrieved to display the history of the electrical activity over the 60-ms period following the inducing stimulus. Yamada, et al display this history in a pictorial fashion by a series of light-emitting diodes, which are individually either turned on or off to correspond to the relative presence or absence of electrical activity in a 2-ms interval.
While the Yamada device represents a practical improvement over laboratory instrumentation, it is unnecessarily complicated and still calls for interpretive skill in understanding the displayed waveform. Moreover, certain muscle and jaw disorders can be diagnosed on the basis of Silent Period measurement without simultaneous latency measurement. Clinicians interested only in such applications have no need for the extra capabilities and concomitant complications of the Yamada instrument.
The success of an electromyographic device as a diagnostic tool in a private medical or dental office depends on its cost and simplicity of operation. The lower the cost of the instrument and the easier it can be applied, the lower will be the expense to the patient.